New insights into membrane fouling by alginate: Impacts of ionic strength in presence of calcium ions

While water chemistry (e.g., ionic strength, calcium concentration and organic foulants) is the primary property of surface water, its effects on membrane fouling in process of membrane-based water production and seawater pretreatment have not well investigated. In this study, fouling behaviors of alginate solutions in presence of different calcium ion concentration and ionic strength levels were investigated. It was found that alginate solutions complexing with 1.5 mM calcium possessed a remarkably high specific filtration resistance (SFR) (above 3.596 × 1015 m kg−1), and the SFR descended with calcium concentration and increased with ionic strength. A series of characterizations suggested that zeta potential, particle size, viscosity and morphology of alginate solutions were close related with foulant layer microstructure and these fouling behaviors. Based on these characterizations, the thermodynamics described by Flory-Huggins lattice theory was proposed to explain the remarkably high SFR of alginate gel for 1.5 mM calcium level. Meanwhile, preferential intermolecular coordination combined with Flory-Huggins lattice theory was suggested to be responsible for the descend trend of SFR with calcium concentration. Furthermore, electrostatic double layer compression effect together with Flory-Huggins lattice theory could well interpret the increase trend of SFR with ionic strength. This study provided the essential mechanisms underlying effects of ionic strength on alginate fouling in presence of calcium ions, and thus deepened understanding of membrane fouling. [Display omitted] •Alginate solutions complexing with 1.5 mM calcium possessed remarkably high SFR.•Alginate SFR descended with calcium concentration and increased with ionic strength.•The remarkably high SFR of alginate can be explained by gel thermodynamics.•Preferential intermolecular coordination was responsible for descend SFR with calcium.•Upward trend of SFR with ionic strength is interpreted by double-layer compression and Flory-Huggins theory.